Piezoelectric transducers of this type are well known as an ultrasonic ceramic microphone in which a resonance characteristic of a piezoelectric element is utilized to resonate at a specific frequency. It is known that the ultrasonic ceramic microphone has relatively high sensitivity at a specific frequency, and thus is conveniently used as a transmitter for generating a predetermined ultrasonic wave or a receiver for receiving such a ultrasonic wave.
In a conventional ultrasonic ceramic microphone, a piezoelectric ceramic oscillator is bonded to the bottom inside of a bottomed cylindrical casing which is reduced in thickness of the bottom portion as compared with that of side portion so as to decrease the influence due to the side supporting, and a terminal board is mounted in an opening of the casing by means of an elastic adhesive. In this case, the shape of the casing is cylindrical and the side portion thereof is thicker than that of the bottom portion on which the piezoelectric ceramic oscillator is disposed. Thus, the substantial mass of the assembly is increased, and the Q of the vibration is also increased. Therefore, the Q of the acoustic resonance is high so that the frequency band is disadvantageously narrowed. Further, since the casing does not have a constant thickness, the machining thereof is costly.
An ultrasonic ceramic microphone which can eliminate said disadvantages is disclosed in Japanese Utility Model Kokai No. 56099/82, in which it comprises a frustoconical casing having a bottom portion at one end and an opening at the other end thereof, a piezoelectric ceramic oscillator bonded to the inner surface of the bottom portion of the casing, and a terminal board mounted on the opening of the casing. Such ultrasonic ceramic microphone may be usually fixed on a suitable mounting member and may be used for transmitting or receiving a ultrasonic wave.
With the ultrasonic ceramic microphone fixed on the mounting member, when the microphone is to be used as a transmitter for transmitting a ultrasonic wave, the oscillation is leaked out through the terminal board to the mounting member so that the output level decreases, and the mounting member is vibrated to produce a noise. When the microphone is to be used as a receiver for sensing a ultrasonic wave, the vibration from the mounting member is propagated through the terminal board to the piezoelectric ceramic oscillator bonded to the inner surface of the casing, and a noise is introduced into the output signal, thereby reducing the signal-to-noise ratio of the microphone.
In the conventional ultrasonic ceramic microphone, usually, the space in the casing sealed by the terminal board is filled with the air. Therefore, the adhesive of the bonding surface of the piezoelectric element is feasibly deteriorated by moisture or oxygen in the air, various aging changes occur in the casing, in which electrode films are rusted, causing the performance to be deteriorated or some other trouble, thereby decreasing the life of the device.
It is appreciated that a base portion such as terminal board in an ultrasonic ceramic microphone has a forced oscillation mode and a free oscillation mode. The former remarkably occurs at the frequency lower than 40 KHz, and the latter at frequency higher than 40 KHz. When the diameter of the base portion is represented by "D", in the forced oscillation mode the base portion bendably vibrates in such a manner that a node lines in the position of 0.40 D to 0.45 D, theoretically, 0.417 D from the center of the base portion, and in the free oscillation mode the base portion vibrates in such a manner that a node lies in the position of 0.30 D to 0.35 D, theoretically, 0.33 D from the center of the base portion.
It is, therefore, an object of this invention to provide a piezoelectric transducer for transmitting or receiving an ultrasonic wave which can insulate a vibration between a piezoelectric element and a mounting member and improve the transmitting sound pressure level and the receiving sensitivity.
Another object of the invention is to provide a piezoelectric transducer for transmitting or receiving an ultrasonic wave in which an influence of the atmosphere can be substantially eliminated, thereby extending the durable life.
A further object of the invention is to provide a piezoelectric transducer for transmitting or receiving an ultrasonic wave which is to be used in a backward view sensor system for a vehicle.